1. Field of the Invention
The present invention relates to a micro electro-mechanical system (MEMS) and a method of manufacturing the same.
2. Description of the Related Art
A Micro Electro-Mechanical System (MEMS) is a device in which electric circuits (control part) and mechanical macrostructures (movable part) are integrated on one substrate. The micro electro-mechanical system is manufactured by using various microfabrication technologies such as semiconductor manufacturing technology and laser processing technology. In various industrial fields such as information and communication, medical treatment, bio, and cars, the micro electro-mechanical system is a key device which is small, highly accurate, and excellent in power saving. A MEMS resonator utilizing the MEMS technology drives an oscillator formed by a silicon micro-fabrication technology and resonates the oscillator by utilizing an electrostatic force and electromagnetic force. Since the oscillator of the MEMS resonator changes it's physical properties, such as a resonance frequency, dependent on variations of temperature, pressure, etc., the oscillator of the MEMS resonator is applied to various oscillatory sensors. The MEMS resonator is applied to, for instance, an infrared sensor in which a resonance frequency of the oscillator varies dependent on stress relaxation of the oscillator due to infrared absorption and thermal expansion. The MEMS resonator is also applied to a pressure sensor utilizing a change of resonance frequency of the oscillator due to strain.
FIGS. 1A and 1B are respectively a perspective view and a cross-sectional view of a force transducer including a MEMS resonator described in U.S. Pat. No. 5,188,983 (document D1). The force transducer includes a beam 34 which functions as an oscillator. The beam 34 is consisted of poly-silicon formed on a silicon substrate 31 having a cavity 32. The beam 34 is covered with an outer shell 40 formed with poly-silicon similarly. As shown in FIG. 1A, edges of the beam 34 are supported by the silicon substrate 31 connected thereto, so that spaces are provided on and below the beam 34. A capacitor is formed by an electrode 43 formed at one edge of the beam 34 and a metallic pad 42 formed on the outer shell 40. An electrode 44 formed at the other edge of the beam 34 is utilized for supplying an excitation signal to oscillate the beam 34. External force applied on the beam 34 makes the resonance frequency of the beam 34 vary. A space surrounding the beam 34 is formed by removing a sacrifical film by means of an etching method. After forming the sacrifical film, an outer shell 40 is formed and patterned.
In document D1, MEMS resonators having structures shown in FIGS. 2A to 2C are also described. The MEMS resonators of FIGS. 2A to 2C is a typical electrostatic capacitance type MEMS. The MEMS resonator of FIG. 2A having a cantilever structure includes a fixed electrode 2 and a movable electrode 1 which functions as an oscillator. These electrodes form a capacitor for accumulating electric charges. The movable electrode 1 vertically oscillates as shown in FIG. 2A, and a change of the electric capacitance corresponding to a change of a gap length between the movable electrode 1 and the fixed electrode 2 is output as an outgoing signal. The MEMS resonator of FIG. 2B having an interleaved structure includes a movable electrode 1 and fixed electrodes 2, both of which form a capacitor for accumulating electric charges. The movable electrode 1 horizontally oscillates as shown in FIG. 2B, and a change of the electric capacitance corresponding a change of area of the capacitor is output as an outgoing signal. The MEMS resonator of FIG. 2B is sensitive because the change of the electric capacitance when the change of area of the capacitor increases at oscillation. However, the MEMS resonator of FIG. 2B needs a large excitation energy due to its large mass. The MEMS resonator of FIG. 2C having a tuning fork structure includes a movable electrode 1 and a fixed electrode 2 both of which form a capacitor for accumulating electric charges. The movable electrode 1 horizontally oscillates as shown in FIG. 2B, and a change of the electric capacitance corresponding to a change of a gap length between the movable electrode 1 and the fixed electrode 2 is output as an outgoing signal. A sensitivity of the MEMS resonator of FIG. 2C is improved by narrowing a gap length between the movable electrode 1 and the fixed electrode 2.
Usually, the movable part such as the movable electrodes is fabricated by forming a sacrifical film as described in document D1. A method of separating the movable part from the substrate by etching sacrifical film is utilized. As the method of etching the sacrifical film, a wet etching method is usually utilized. In the wet etching method, a gap between the movable part and fixed part is narrowly formed. Therefore, there is a possibility that sticking phenomena may occur due to surface tension of an etchant in a drying process after etching the sacrifical film. The sticking phenomena means that the movable part adsorbs to fixed part due to the surface tension of the etchant. The sticking phenomena tend to occur as the gap length between the movable part and fixed part decreases. Therefore, it is difficult to stably manufacture a MEMS resonator with a narrow gap such as the tuning folk structure as shown in FIG. 2C at a high yield rate.